Allow changing existing modes and adding new ones via `modes`
configuration key.
`modes` accepts a JSON object roughly described by the following type
```typescript
type BarMode = {
layer: 'bottom' | 'top' | 'overlay';
exclusive: bool;
passthrough: bool;
visible: bool;
};
type BarModeList = {
[name: string]: BarMode;
};
```
and will be merged with the default modes defined in `bar.cpp`.
Note that with absence of other ways to set mode, only those defined in
the `sway-bar(5)`[1] documentation could be used right now.
[1]: https://github.com/swaywm/sway/blob/master/sway/sway-bar.5.scd
b6d0a4b63f